The present invention relates generally to devices for transmitting ultrasonic energy waves and, more particularly, to methods and apparatus for performing ultrasonic wave interferometry and for using ultrasonic wave interferometers for sensing physical phenomena.
As used herein, an ultrasonic wave interferometer is defined as an ultrasonic energy transmitting device including a pair of separate ultrasonic energy paths having substantially equal transmission path characteristics when the device is in a neutral condition relative to a physical phenomenon to be sensed. At least one of the paths is treated to respond to the physical phenomenon to be sensed such that its transmission path characteristics change as a result of the phenomenon to vary an interference pattern defined at the output of the device. Although the wave energy used in the present invention is ultrasonic, the term acoustic has come to be equivalent in the art and, accordingly, the two terms will be used interchangeably herein.
A wide variety of processes are controlled in response to sensing chemical concentrations, visual as well as other electromagnetic radiation, magnetic field strength, acceleration, angular velocity, applied forces, and other parameters which will be collectively referred to herein as physical phenomena. Such sensing is performed by devices which convert the physical phenomenon being sensed into electrical signals. The resulting electrical signals can then be used to control a related process or to warn or inform personnel performing a sensing or monitoring operation. It is apparent that the automation and accurate control of many processes depends on the availability of reliable sensors.
Preferably, sensors are compact, robust and inexpensive, particularly in applications where multiple sensing operations must be reliably performed in hostile environments for proper operation and control of complex operating systems. An example of such a control environment is a modern motor vehicle. Such vehicles include a variety of operating systems which must be accurately monitored and controlled for safe and effective operation of the systems and hence the vehicles.
One currently available sensor arrangement utilizes an ultrasonic energy transmission device wherein two isolated delay lines are incorporated into a pair of substantially identical oscillators. Accordingly, the device, including a substantial amount of associated circuitry, is referred to as a dual delay line oscillator circuit.
The amplitudes of oscillation of the oscillators are preferably controlled by means of automatic gain control circuits (AGC's). A physical phenomenon to be sensed, such as a gas concentration around the sensor, is monitored by coating one of the delay lines with a material (chemical or biochemical interface) which alters the transmission characteristics of the delay line in response to the phenomenon. Alteration of the transmission characteristics of the delay line in turn alters the frequency of oscillation of the oscillator circuit including the delay line. The oscillator output signals are mixed and passed through a low pass filter to generate a signal having a frequency which is representative of the phenomenon.
While the dual delay line oscillator circuit can reliably sense gas concentrations and other physical phenomena, it requires substantial circuitry in addition to the device or devices which define the acoustic or ultrasonic delay lines. There is thus a need for an improved and simplified arrangement which can be used to measure physical phenomena without requiring the amount of support circuitry required by the dual delay line oscillator circuit.